1. Field of the Invention
The present invention relates to a transmission circuit usable for communication devices such as mobile phones and wireless LAN devices, and in particular to a transmission circuit operable at a high efficiency and a low distortion and a communication device using the same.
2. Description of the Background Art
Communication devices such as mobile phones and wireless LAN devices are required to guarantee a linearity of a transmission signal in a wide power amplification range and also to operate at low power consumption. For such a communication device, a transmission circuit operable at a high efficiency and a low distortion is used. Hereinafter, conventional transmission circuits will be described.
Conventionally, a transmission circuit for controlling the voltage to be supplied to an amplification section in accordance with the magnitude of an envelope of a modulated signal is disclosed (see, for example, non-patent document 1). FIG. 29 is a block diagram showing an exemplary structure of a conventional transmission circuit 900. In the conventional transmission circuit 900, a signal generation section 901 generates an in-phase signal and a quadrature-phase signal (herein after, such an in-phase signal and a quadrature-phase signal will be referred to as “I and Q signals”), which are quadrature signals. The I and Q signals are input to a vector modulation section 902. The vector modulation section 902 performs vector modulation on the I and Q signals and outputs the resultant signal as a modulated signal. As the vector modulation section 902, a quadrature modulator is typically used. The modulated signal which is output from the vector modulation section 902 is input to a detection section 904 and an amplification section 906 via a divider 903.
The detection section 904 detects the magnitude of the envelope of the modulated signal and outputs the detection signal to a regulator 905. The regulator 905 is supplied with a DC voltage from a power supply terminal 907. The regulator 905 supplies a voltage in accordance with the magnitude of the envelope of the modulated signal, which is detected by the detection section 904, to the amplification section 906. The amplification section 906 amplifies the modulated signal in accordance with the voltage supplied from the regulator 905. The modulated signal amplified by the amplification section 906 is output from an output terminal 908 as a transmission signal. In this way, the conventional transmission circuit 900 controls the voltage to be supplied to the amplification section 906 in accordance with the magnitude of the envelope of the modulated signal, and thus prevents the distortion characteristic of the modulated signal from being deteriorated by the amplification section 906 especially in the case where the envelope of the modulated signal which is output from the vector modulation section 902 is large.
A conventional transmission circuit for controlling the transmission power required for communication with a base station at each slot time in accordance with the W-CDMA system is disclosed (see, for example, patent document 1). FIG. 30 is a block diagram showing an exemplary structure of a conventional transmission circuit 910. In the conventional transmission circuit 910, a table referring section 913 has stored therein information on a power supply voltage at which an amplification section 916 always operates at a maximum efficiency, for each magnitude of the transmission power. A regulator 914 receives a voltage corresponding to an average power, which is obtained by averaging the transmission power in each slot time, via an input terminal 911 and the table referring section 913. The regulator 914 is also supplied with a DC voltage from a power supply terminal 915.
The regulator 914 reads the information on the power supply voltage corresponding to the input average power from the table referring section 913, and thus controls the voltage to be supplied to the amplification section 916. The amplification section 916 amplifies a modulated signal which is input via an input terminal 912 in accordance with the voltage supplied from the regulator 914. The modulated signal amplified by the amplification section 916 is output from an output terminal 917 as a transmission signal. In this way, the conventional transmission circuit 910 controls the voltage to be supplied to the amplification section 916 based on the average transmission power in each slot time, and thus operates at a high efficiency and a low distortion.
A conventional transmission circuit for generating a transmission signal using a modulation system such as a quadrature modulation system or the like is known (herein after, such a transmission circuit will be referred to as a “quadrature modulation circuit”). The quadrature modulation circuit is well known and will not be described herein. Non-patent document 2 discloses a conventional transmission circuit for outputting a transmission signal having a higher linearity at a higher efficiency than the quadrature modulation circuit. FIG. 31 is a block diagram showing an exemplary structure of a conventional transmission circuit 920 disclosed in non-patent document 2. As shown in FIG. 31, the conventional transmission circuit 920 includes a signal generation section 921, an angle modulation section 922, a power supply terminal 923, a regulator 924, an amplitude modulation section 925, and an output terminal 926. The signal generation section 921 generates an amplitude signal and a phase signal. The amplitude signal is input to the regulator 924. The regulator 924 is supplied with a DC voltage Vb from the power supply terminal 923. The regulator 924 supplies a voltage Vc controlled in accordance with the amplitude signal input thereto to the amplitude modulation section 925. The phase signal is input to the angle modulation section 922. The angle modulation section 922 performs angle modulation on the input phase signal and outputs an angle modulated signal. The angle modulated signal is input to the amplitude modulation section 925. The amplitude modulation section 925 performs amplitude modulation on the angle modulated signal with the voltage Vc supplied from the regulator 924, and outputs a modulated signal obtained as a result of angle modulation and amplitude modulation. The modulated signal is output from the output terminal 926 as a transmission signal. The transmission circuit 920 operable in this manner is referred to as a polar modulation circuit.
With the conventional transmission circuit 900 (see FIG. 29), the voltage supplied to the amplification section 906 is continuously varied by the envelope of the signal processed with vector modulation by the vector modulation section 902. Therefore, the amplification section 906 may possibly operate in a non-linear region (saturated region) as well as the linear region. For example, the amplification section 906 needs to be supplied with a voltage of a certain level or greater in order to maintain the linearity of the transmission signal. If the amplification section 906 is not supplied with a voltage of such a certain level or greater, the transmission circuit 900 cannot maintain the linearity of the output signal. Also when the envelope of the signal processed with vector modulation temporarily exceeds the linear region of the amplification section 906, the conventional transmission circuit 900 cannot maintain the linearity of the output signal.
The conventional transmission circuit 910 (see FIG. 30) controls the voltage to be supplied to the amplification section 916 using the average power in each slot time. Therefore, the transmission circuit 910 cannot deal with a sudden change in the power of the transmission signal and may not always operate at a high efficiency and a low distortion. For example, even when the average power in each slot time is high but is lower in a time period shorter than the slot time, the conventional transmission circuit 910 may possibly supply a voltage corresponding to the average voltage in each slot time to the amplification section 916. As a result, the power is wasted in the amplification section 916 and a high efficiency operation is not necessarily realized. By contrast, when the average power in each slot time is low but is higher in a time period shorter than the slot time, the power supplied to the amplification section 916 is not sufficient and thus the transmission signal may be distorted.
The conventional transmission circuit 920 (see FIG. 31) controls the voltage Vc which is output from the regulator 924 with the amplitude signal. Therefore, when the magnitude of the amplitude signal is small, the power lost by the regulator 924 is large and thus the efficiency of the transmission circuit is decreased. FIG. 32 shows a problem with the conventional transmission circuit 920. Referring to FIG. 32, a difference between the voltage Vb supplied from the power supply terminal 923 to the regulator 924 and the voltage Vc supplied from the regulator 924 to the amplitude modulation section 925 is the loss caused at the regulator 924. When the magnitude of the amplitude signal is small, the difference between the voltages Vb and Vc is large, which decreases the efficiency of the transmission circuit.
In order to reduce such a loss, patent document 2 discloses a conventional transmission circuit 930 in which a regulator includes a series regulator and a switching regulator. FIG. 33 is a block diagram showing an exemplary structure of the conventional transmission circuit 930 disclosed in patent document 2. As shown in FIG. 33, the conventional transmission circuit 930 includes a regulator 931 and an amplitude modulation section 932. The regulator 931 includes a switching regulator 933 and a series regulator 934. FIG. 34 shows an operation of the regulator 931 in the conventional transmission circuit 930. Referring to FIG. 34, the switching regulator 933 is supplied with power information which represents the magnitude of the power of a transmission signal to be output.
The switching regulator 933 supplies a voltage Vs controlled in accordance with the power information to the series regulator 934. Since the frequency of the power information is lower than that of the amplitude signal, the switching regulator 933 can operate at a high efficiency. The series regulator 934 supplies a voltage Vc controlled in accordance with the magnitude of the amplitude signal to the amplitude modulation section 932. In this way, the switching regulator 933 supplies the voltage Vs controlled in accordance with the power information to the series regulator 934, and thus the conventional transmission circuit 930 reduces the loss in the regulator 931.
However, with the conventional transmission circuit 930 (see FIG. 33), the frequency of the power information is lower than that of the amplitude signal. Therefore, the operation of the switching regulator 933 does not follow a change in the amplitude signal. For this reason, the loss in the series regulator 934 cannot be sufficiently reduced. FIG. 35 shows a problem of the conventional transmission circuit 930. As shown in FIG. 35, with the conventional transmission circuit 930, there is still a difference between the voltage Vs supplied from the switching regulator 933 to the series regulator 934 and the voltage Vc supplied from the series regulator 934 to the amplitude modulation section 932. In a consequence, the conventional transmission circuit 930 cannot sufficiently reduce the loss in the series regulator 934, and the efficiency of the transmission circuit is still decreased.
Patent document 1: Japanese Laid-Open Patent Publication No. 11-251934
Patent document 2: U.S. Pat. No. 6,636,112
Non-patent document 1: P. B. Kenington, “High Linearity Rf Amplifier Design”, Artch House Microwave Library, USA, January 2000, pp. 426-512
Non-patent document 2: F. H. Raab, et al., “High-efficiency L-band Kahn-technique transmitter”, 1998, IEEE MTT-S Int. Microwave Symp. Dig.